[openstack-dev] [nova] nova-manage db archive_deleted_rows broken

Jay Pipes jaypipes at gmail.com
Fri Oct 9 17:03:08 UTC 2015

On 10/07/2015 11:04 AM, Matt Riedemann wrote:
> I'm wondering why we don't reverse sort the tables using the sqlalchemy
> metadata object before processing the tables for delete?  That's the
> same thing I did in the 267 migration since we needed to process the
> tree starting with the leafs and then eventually get back to the
> instances table (since most roads lead to the instances table).

Yes, that would make a lot of sense to me if we used the SA metadata 
object for reverse sorting.

> Another thing that's really weird is how max_rows is used in this code.
> There is cumulative tracking of the max_rows value so if the value you
> pass in is too small, you might not actually be removing anything.
> I figured max_rows meant up to max_rows from each table, not max_rows
> *total* across all tables. By my count, there are 52 tables in the nova
> db model. The way I read the code, if I pass in max_rows=10 and say it
> processes table A and archives 7 rows, then when it processes table B it
> will pass max_rows=(max_rows - rows_archived), which would be 3 for
> table B. If we archive 3 rows from table B, rows_archived >= max_rows
> and we quit. So to really make this work, you have to pass in something
> big for max_rows, like 1000, which seems completely random.
> Does this seem odd to anyone else?

Uhm, yes it does.

 > Given the relationships between
> tables, I'd think you'd want to try and delete max_rows for all tables,
> so archive 10 instances, 10 block_device_mapping, 10 pci_devices, etc.
> I'm also bringing this up now because there is a thread in the operators
> list which pointed me to a set of scripts that operators at GoDaddy are
> using for archiving deleted rows:
> http://lists.openstack.org/pipermail/openstack-operators/2015-October/008392.html
> Presumably because the command in nova doesn't work. We should either
> make this thing work or just punt and delete it because no one cares.

The db archive code in Nova just doesn't make much sense to me at all. 
The algorithm for purging stuff, like you mention above, does not take 
into account the relationships between tables; instead of diving into 
the children relations and archiving those first, the code just uses a 
simplistic "well, if we hit a foreign key error, just ignore and 
continue archiving other things, we will eventually repeat the call to 
delete this row" strategy:


I had a proposal [1] to completely rework the whole shadow table mess 
and db archiving functionality. I continue to believe that is the 
appropriate solution for this, and that we should rip out the existing 
functionality because it simply does not work properly.


[1] https://review.openstack.org/#/c/137669/

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